This is a single-arm, multicenter, expanded access study of Iodine-131 Anti-B1 Antibody for patients with relapsed or refractory low-grade or transformed low-grade non-Hodgkin's B-cell lymphoma [NHL]. The primary objective of this study is to make Iodine-131 Anti-B1 antibody more broadly available to patients. Secondary endpoints of the study will be to obtain additional information on the efficacy and safety of Iodine-131 Anti-B1 Antibody. There will be two dosing phases in this study and then up to two years of follow-up. In the first phase, termed the "dosimetric dose," patients will receive an infusion of unlabeled Anti-B1 antibody [450mg.] over 60 minutes immediately followed by a 30-minute infusion [including a 10-minute flush] of Anti-B1 Antibody [35mg] which has been trace-labeled with 5 mCi of Iodine-131. Whole body counts using a gamma camera will be obtained 3 times between Days 0 and 7 following the dosimetric dose. The whole body counts will be used to determine a patient-specific mCi dose of iodine-131 calculated to deliver the desired total body dose of radiation [either 65 or 75 cGy]. The second phase, termed the "therapeutic dose, is administered 7-14 days after the dosimetric dose. Patients will receive a 60 minute infusion unlabeled Anti-B1 Antibody [450 mg.] immediately followed by a 30-minute infusion [including a 10-minute flush] of Anti-B1 Antibody [35 mg.] labeled with the patient-specific dose of Iodine-131 [median dose in previous studies was approximately 85 mCi]. Patients who are obese will be dosed based upon 137% of their calculated lean body mass. Patients will be treated with either saturated solution potassium iodide [SSKI], Lugol's solution, or potassium iodide tablets starting at least 24 hours prior to the first infusion of the Iodine-131 Anti B1 Antibody [i.e., dosimetric dose] and continuing for 14 days following the last infusion of Iodine-131 Anti-B1 Antibody [i.e., therapeutic dose]. Hematology, blood chemistry and adverse experiences will be followed for 13 weeks. Response evaluations will be performed at weeks 13 and 25 and then every 6 months until disease progression, patient death, or for two years, whichever occurs first. Thyroid function will also be followed.